- "Hm? What's this? A voice beckoned you here? I see. So the time has come. I shall now tell you all I know about the arduous trial that awaits you. The sword you have reunited with...the Master Sword...it has yet to realize its true splendor. This trial shall test your courage to ensure you are worthy of this splendor. It is called the Trial of the Sword. It will challenge you as you have never been challenged before. All of your weapons and armor will be sealed away. You must face your enemies without the tools you've come to rely on. Once you have tempered your soul and proven your wisdom and courage by completing the Trial of the Sword...the Master Sword shall truly be yours! ...Hm. Unless my eyes deceive me, you seem quite willing to take on this trial. A hero through and through! Now, Link... Place the Master Sword in its pedestal."
- — The Great Deku Tree
EX Trial of the Sword is a side quest from The Legend of Zelda: Breath of the Wild. It is unlocked upon installation of the DLC Pack "The Master Trials". Link must have already visited the Korok Forest and obtained the Master Sword. After loading a data save to continue a playthrough, Link receives a floating message requesting him to go to Korok Forest and listen to the The Great Deku Tree in order to receive a new challenge. The quest is thus triggered as well as a series of new side quests including the "EX Strange Mask Rumors", "EX Teleportation Rumors!", and the four Misko's "EX Treasure: Ancient Mask", "Fairy Clothes", "Twilight Relic" and "Phantasma".
The quest giver's identity is unknown at this time but named "???" at the mission menu.
The voice mentions a trial and advises Link to speak with the The Great Deku Tree at Korok Forest to take on the challenge. According to The Great Deku Tree, the Master Sword's power has to be awakened. When Link approaches the pedestal, he requests him to place the sword back in its pedestal and to face the Trial of the Sword, which takes place in the Sword Monks' Shrine.
Similar to what occurs at arriving on Eventide Island for the first time in search of the Shrine during "Stranded on Eventide", Link will not be permitted to use any of his weapons, armor, items, or gear that he has collected from outside of the trial and must rely on what he collects inside of the trial for survival. Link is also barred from using the Travel Medallion within the trial and resuming progress from that floor, should Link leave for whatever reason. When Link has gone through all forty five floors, at the end he will come across the "Sword Monks" who are seven unnamed Sheikah Monks who will return the Master Sword back to its original splendor.
In order to clear a floor, Link must defeat the enemies of that floor with whatever he has on him at the time so he can teleport to the next floor. If Link departs from the trial before he clears all forty-five floors, he must restart the trial all the way from the first floor. However the trial is separated into three sections and when he completes a section Link can stop and return to take on the others later. The first section is the easiest and once completed the Master Sword's base attack power is increased to 40. The second section is moderately difficult and once completed the Master Sword's base attack power is increased to 50. The third and final section is the hardest and once completed unlocks the Master Sword's "true" power state permanently increasing its attack power to 60 and increasing its durability.
Unlocking the "True" Master Sword
Upon completing this trial, Link will find a room with seven Sword Monks who restore the Master Sword's true power. Once Link removes the Master Sword from its pedestal within the room, the Master Sword's "True" full powered glowing state will be unlocked permanently, thus allowing Link to use it at all times. The Master Sword's true power state doubles the Master Sword's 30 base attack to 60 and increases its durability.
It does not make the Master Sword truly "unbreakable" as it still loses its charge and enters its cool down phase (the duration of which remains the same), though its increased durability allows it to be used more and its allows Link to use its full power at all times, instead only when he's facing corrupted enemies, Scourges of the Divine Beasts, and Ganon himself.
It is unclear who the mysterious voice that gives Link the trial though their are two strong possibilities. The first is one of the sword monks is responsible as they oversee the trial itself and other Sheikah Monks can communicate with Link from afar like Korgu Chideh Shrine when he gives "Stranded on Eventide". The second is the Goddess Hylia who played a major role in the Master Sword's creation and gives Link the Shrine Quests for the Springs of Power, Wisdom, and Courage. Hylia is also implied to have tasked the Sword Monks with their duty of testing Link and presumably blessed them with the power to unlock its true splendor permanently once Link proved himself worthy by completing the trial. The Great Deku Tree is also implied to be working with Hylia and the Monks explaining his knowledge of the sword's divine origins and supported by Hylia's history of being allied with other deities and guardian spirits to protect the land from evil. As a Goddess Hylia would have little trouble communicating with Link to inform him of the trial.